It is known to cut and retrieve wellheads on abandonment using a single-trip tool which combines a casing cutter and a grapple for engaging the wellhead. An example of such an arrangement is shown in our published International Patent Application No W091/02138. That application, and other prior art, describes an arrangement in which the cutter is driven by rotating the entire string from the surface platform. While this is satisfactory in many cases, problems can arise especially in deep water from poor alignment of the wellhead with the rotary table or from the torsional elasticity of the string.
It is also known to drive the cutter by means of a mud motor, with a view to avoiding the foregoing problems. This has been done, however, using a standard mud motor connected above the wellhead, driving the cutter via a mandrel extending through the grapple. Such an arrangement has a number of disadvantages. The pull which can be exerted through the mud motor is limited typically to about 200,000-250,000 lbs, whereas a significantly higher force is desirable for removing the wellhead after cutting. Also, the swivel connections normally provided above and below the mud motor can be damaged by transverse loads, and such loads are common in wellhead retrieval due to axial misalignment or whipping effects in the string, especially in deep water.